Measuring of fluid density

ABSTRACT

The density of a fluid is measured employing apparatus including a cylinder immersed in the fluid, together with driving and pickup means operating to resonate the cylinder at its natural frequency as affected by the fluidic medium; the apparatus described has the drive and pick-up coils located external to the resonating cylinder so that they are free of the fluidic medium.

O R: United States Patent 1 [111 3,763,692 Agar ct. 9, 1973 MEASURING 0FFLUID DENSITY [56] References Cited [75] Inventor: Joram Agar,Hampshire, England UNITED STATES PATENTS [73] Assignee; Joram Agar &Company Limited 3,516,283 6/1970 Abbotts 73/32 x Hampshire, EnglandPrimary ExaminerRichard C. Quelsser [22] Filed: Apr. 5, 1972 AssistantExaminer-Arthur E. Korkosz [211 Appl 241,470 Attorney-John W. Logan, Jr.

Re'iated US. Application Data [57] ABSTRACT [63] 5522; 2:3 of July 1968The density of a fluid is measured employing apparatus including acylinder immersed in the fluid, to- [30] Foreign Application PriorityData gether with driving and ick-up means operating to resonate thecylinder at its natural frequency as af- July 26, Great Britain ct theme e apparatus described has the drive and pick-up coils locatedexternal to the resonating cylinder so that they are free of the fluidic[58] Field of Search 73/30, 32

15 Claims, 5 Drawing Figures PATENTED 9% 3.763 .692

' sum 1 or 3 [NVEN'TOR JORAM AGAR ATTORNEY PATENTEU 91975 3. 763 .692

FIG. 5

I N VEN TOR.

JORAM AGAR ph-K W ATTORNEY MEASURING F FLUID DENSITY This is acontinuation of application, Ser. No. 744,711, filed July 15, 1968, nowabandoned.

This application calims the priority date of July 26, 1967, on the basisof British Application, Ser. Number 34378/67. I

This invention concerns a method of, and apparatus for measuring thedensity of a fluid.

Accurate measurement of the density of a fluid is essential in a numberof different fields of technology, e.g. in metering fluids like naturalgas, or steam, or aircraft fuel, or in blending operations in foodtechnology, gas chromatography, bio-chemistry and many other fields.

According to the present invention there is provided a method ofmeasuring the density of a fluid, said method including passing thefluid through the interior of a hollow body mounted within a rigidsupport member, locating drive means and detecting means externallyaround, but out of contact with, said body, passing some of said fluidaround the outside of said body in contact therewith, but out of contactwith said drive means and with said detecting means so as substantiallyto equalise the fluid pressures acting on said body, exciting the saidbodyby said drive means to vibrate at a resonant'frequency whereby togenerate in said detecting means a signal which is, in operation,frequencymodulated in dependance upon the density of said fluid passingthrough said body, and monitoring said signal.

Preferably, said drive means in operation excites the said body tovibrate in a circumferential mode of vibrations.

In one preferred embodiment, both said drive means and said detectingmeans are electromagnetic means located around the exterior of said bodysuch that, in operation, the flux paths thereof substantially do notoverlap each other and respectively extend into said body over only asmall region thereof relative to the length of said body so as to reducecross-coupling between said drive means and said detecting means.

Alternatively, the vibrations of said body may be excitedelectrostatically; or, in a further alternative, the vibrations of saidbody may be excited electroacoustically. Thus the vibrations may beexcited piezoelectrically, or magneto-strictively.

According to another aspect of the present invention,

there is provided apparatus for measuring the density of a fluid, saidapparatus including a rigid support member within which is mounted ahollow body adapted to have said fluid passing through its interior,drive means for exciting said body to vibrate at a resonant frequency,detecting means for detecting a signal representative of the frequencyof said vibrations, which signal is frequency-modulated in operation independence upon the density of said fluid, said drive means and saiddetecting means being so mounted on said support member as to allow saidfluid to pass externally around said body in contact therewith, but outof contact with said drive means and said detecting means, andmonitoring means for monitoring said signal.

Said body is preferably a tube and may be of substantially circularcross-section. Moreover, said body may be provided at each end thereofwith flanges so as to render said body substantially dumb-bell shaped.

In a preferred embodiment, said flanges are provided with aperturesthrough which said fluid may pass externally around said body.

Both said drive means and said detecting means may be electromagneticmeans located around the exterior of said body such that, in operation,the flux paths thereof substantially do not overlap each other andrespectively extend into said body over only a small region thereofrelative to the length of said body so as to reduce cross-couplingbetween the said drive means and the said detecting means.

In an alternative, said drive means and said detecting means mayrespectively be constituted by electroacoustic transducers for vibratingthe said body.

In a futher alternative, said drive means and said detector means may becapacitor plates, the said body constituting the other plate for thedrive capacitor and for the detector capacitor for the electrostaticexcitation of vibrations in said body.

Preferably there is provided a locating pin for connecting said supportmember to the said body such that removal and replacement of said bodymay be effected without re-adjusting the relative positions of the saidbody and the said support member.

A core may be provided within the interior of the body so as to reducethe cross-sectional area of fluid flow therewithin.

Said support member may be provided with an end portion engageable withan adaptor mounted on a handle for use of the apparatus in relativelyinaccessible locations.

Preferably, said support member is a rigid casing surrounding said bodyalong at least the major part of the length of said body.

Filter means are preferably provided for filtering the fluid prior toits passing in contact with said body.

The invention will be described,merely by way of example, with referenceto the accompanying drawings, in which:

FIG. 1 is a side view, partly in section, of the apparatus for measuringthe density of a fluid in accordance with the present invention,

FIG. 2 is an end view of the apparatus shown in FIG. 1, l

FIG. 3 is a core member usable in conjunction with the apparatus shownin FIG. 1 in certain applications,

FIG. 4 is a view somewhat similar to FIG. 1, but showing a modified formof the apparatus fitted into an elbow of a fluid pipe line, and- FIG. 5is a side view, partly in section, of the apparatus shown in FIG. 1, butmodified to be used as a probe.

Referring first to FIGS. 1 and 2, there is shown an apparatus formeasuring the density of fluid which comprises a rigid, hollow,cylindrical casing 10 having threads 11 at both its ends for fitting toa pipe-line or other location where the density of a fluid is to bemeasured. The casing 10 completely surrounds a substantially cylindricalthin-walled body 12 mounted within its interior with a gap therebetween,the said body 12 being also hollow and of circular internalcross-section. Each end of the body 12 is formed with a flange 13 so asto give the body 12 a substantially dumb-bell shape. Whereas the rigidcasing 10 is made of a non-magnetic material such as aluminium orstainless steel, the body 12, hereafter referred to as sensing element12, is made of a magnetic material which exhibits only small variationsin natural frequency with changes of temperature.

One such material is sold under the Trade Name ELIN- VAR.

As can be seen, the sensing element 12 is mounted within the rigidcasing by means of lock-rings 14 between which is mounted a filterelement 15. The sensing element 12 is thereby readily detachable fromthe casing 10. FIG. 2 also shows clearly that the flanges 13 arepart-annular and not fully annular.

In order to ensure that the sensing element 12 and the rigid casing 10are accurately positioned with respect to each other, a lock pin 16 isprovided to interconnect the sensing element 12 and the casing 10 whichpin 16 engages in an appropriate aperture formed in the sensing element12. In this way it is ensured that repeated removal of the sensingelement 12, e.g. for cleaning purposes, and consequently the repeatedreplacement thereof within the casing 10 will not necessitate asjustmentof the remainder of the apparatus described below.

It will be appreciated that when, in operation, one end of the casing 10is exposed to fluid flow, the fluid will enter into the interior of thesensing element 12, and owing to the part-annular shape of the flangesl3, fluid will also flow around the sensing element 12 within the casing10 and through the said gap. Thus, in operation, the fluid pressuresacting on the thin cylindrical wall of the sensing element 12 will beequal. If it is desired to make the flanges 13 fully annular, apertures18 may be provided therein to allow circulation of the fluid outside thesensing element 12.

Mounted at approximately the mid-portion of the casing 10 are two coils19, 20. As can be seen, in the radial plane of the coils 19, 20, whichare off-set from each other by l80, the casing 10 is very thin, so thatthe coils are as near as possible to the sensing element 12, yet thethin annular portion of the casing 10 will ensure that the fluid flowwithin and around the sensing element 12 is always out of contact withthe coils 19 and 20. As seen in the drawings, the windings of the coilare in a plane substantially at right angles to the plane of thedrawing, and the core of the coil has three poles. This arrangementensures that when these coils are connected in an appropriate electricalcircuit, the flux paths therefrom do not overlap each other, and extendinto the sensing element 12 over only a very small portion thereofcompared to the total length of the sensing element 12 whereby to reduceor eliminate cross-coupling between the coils 19 and 20. The coil 19 maybe a drive coil and the coil 20 may be a detector or pick-up coil andthey are arranged and maintained in operation at a 90 phase differencewhich results in an arrangement in which viscosity effects aresubstantially reduced or eliminated.

The coils 19 and 20 are secured in place of the casing 10 by means of afiller element 21.

Mounted on the exterior of the casing 10 is an amplifier 22 to which areelectrically connected both the drive coil 19 and the detector coil 20.The amplifier 22 itself is connected to a small dc power source 23, e.g.a 12 volt battery, and the output circuit of the amplifier is connectedto a frequency meter 24.

The operation of the apparatus is as follows:

The rigid casing 10 with the sensing element 12 securely mounted thereinis exposed to fluid flow passing therethrough. With the power source 23connected to the amplifier 22, natural resonant vibrations will be setup and maintained in the sensing element 12 by virtue of feed-back fromthe drive coil 19 to the detector or pick-up coil 20. The vibrations areinitiated by mechanical noise transmitted to the sensing element 12 orby electrical noise occurring in the drive coil 19 when the amplifier 22is switched into action. The end flanges 13 of the sensing element 12will act as nodes for the vibrations of the sensing element 12 and in apreferred embodiment, these vibrations are in the circumferential modeof vibrations, that is to say they are natural bell-like vibrations.Naturally, the frequency of the current supplied to the drive coil 19 issubstantially the same as the predominant frequency of the resonantvibrations of the sensing element 12.

The frequency meter 24 may be of any conventional design capable ofcovering the range of predominant frequencies anticipated for theoperation of the apparatus, and may be calibrated to read directly inunits of density from a graph previously calibrated and prepared throughthe use of fluids of standard or known densities.

In view of the fact that the pressure of the fluid across thecylindrical wall of the sensing element 12 is substantially equalised,the frequency of vibrations of the sensing element 12 is dependentprimarily upon the density of the fluid flowing therethrough, thetemperature effect being rendered substantially small or negligible bythe use of the material of the sensing element 12 having a very lowcoefficient of thermal elasticity.

In fact, the materials of the casing 10 and the sensing element 12 areso chosen that operational pressures of up to l0,000 p.s.i. areacceptable.

It will be appreciated that when the sensing element 12 is set inoscillation in the circumferential mode, and maintained in oscillationby the amplifier 22, the fluid, the density of which is to be measured,is set in oscillation both on the inside and on the outside of thesensing element, and therefore the frequency of oscillation of thesensing element depends on its stiffness and the total oscillating mass,i.e. the walls of the sensing element and of the surrounding fluid. Anincrease in the density of the fluid lowers the frequency of oscillationwhich is monitored by the frequency meter 24. The actual relation iseasily derivable by theory and it is given below.

where f is the frequency at density D, f, is the frequency at zerodensity (vacuum) and K is a constant depending on the diameter andthickness of the wall of the sensing element 12.

The arrangement of the drive means and the detector means of theillustrated embodiment has a number of very important advantages.Firstly, by locating the coils or electo-mechanical drivers out ofcontact with the fluid, the density of which is to be measured, thesafety of the operation of the device is enhanced. Clearly if the fluidin question is an ignitable gas or liquid, the importance of having thecoils out of contact cannot be overstressed.

The arrangement of the straight-through flow is helpful in avoiding anydirt or other obstacle being entrapped and also the sensing element 12can be easily removed for cleaning and inspection. This might well be avery important consideration when the apparatus according to the presentinvention is used in the food industry. Clearly the straight-throughflow is also made possible by the fact that the coils are locatedoutside of the sensing element 12.

The present invention also avoids the use of solenoids for driving thevibrations because that gives rise to long magnetic flux paths anddictates that the pole pieces must be in close proximity of the sensingelement. This is a' source of unreliable operation, since this smallspacing acts as a trap for moisture and iron dust, which are practicallyimpossible to eliminate and which in time will stop the vibration of thesensing element.

The flux pattern of the E-type coils of the present invention is suchthat this path is very short and the gap therefore can be increasedbetween the coil and the sensing element itself. This enables themetering of fluids of high viscosity, and a much longer period oftrouble-free operation.

Yet a further advantage of the illustrated embodiment is a gain inaccuracy and the independence of the frequency of oscillation ofvariations in viscosity of the fluid being measured. In somearrangements using solenoids, because of the long uncontrolled fluxpaths between the two coils, inevitably a certain amount of flux linkingwill occur. The amount of flux linked depends upon the permeability ofthe sensing element which is temperature-dependent. Hence theoverallphase shift and frequency of oscillation will vary withtemperature, and particularly so with high viscosity fluids. Byshortening the flux paths and substantially avoiding crosscoupling thisdefect is substantially eliminated.

A further important advantage in maintaining the phase shift accurate isthe avoidance of mode jumping." By mode jumping" is meant the vibrationof the system occurring at not only the fundamental harmonic frequency,but also at the higher harmonics. The higher the tendency of the systemto jump modes, the lower is the useful range of the instrument. Thus, inthe present invention, by reducing mode jumping," a much larger changein operation of the frequency of oscillation may be tolerated.

A still further advantage is that'the materials of the apparatus are sochosen that the apparatus has a high mechanical 0 thereby rendering itsubstantially immune to vibration and allowing it to be mounted in anyposition.

The apparatus may be fitted to a pipe line in a matter of minutes and itis very easy to attach or detach. It offers very little resistance toflow and because of the amplitude of the vibrations is very small, quiteconsiderably large bubbles in liquids can be tolerated.

In certain applications, such as gas chromatography, and biochemistry,it may be desired to reduce the volume of the fluid flow within thesensing element. This may be achieved by the use of a core member30.such as is shown in FIG. 3. As can be seen the core member 30 isprovided at one end with an annular flange 31 to replace a lock ring 14.The flange 31 is provided at its radially outer end with threads formounting centrally within the interior of the sensing element 12.

Referring now to FIG. 4, there is shown a pipe line 35 having an elbowbend therein, the flow of fluid being shown by a large arrow 36. Thedensity measuring apparatus is substantially similar to that shown inFIG. 1, and like parts have been allotted like reference numbers. Itwill be observed that the apparatus is fitted within the pipe line 35 bymeans of a flange 34 which is attached to the pipe line by means of twocapscrews 38 which are internally undercut to accommodate a sealing ring37. The flange 34 has the amplifier 22 secured thereto, the power source23 and the frequency meter 24 not being shown on this Figure.

In this embodiment, however, instead of a fully annular rigid casing 10,there is provided a substantially fork-shaped support member 40 which isnot annular and merely serves to mount the sensing element 12 and thecoils 19 and 20. As can be seen the leads from the coils pass throughbores in the fork-shaped support member 40 and similarly pass throughwith aligned bores in the flange 34.

The inlet end of the apparatus is provided with a gauze element 35. Thefluid penetrating into the interior of the sensing element 12 passescompletely through the element and then passes round the outside thereofto leave through a plurality of holes or slots 41. Similarly, the fluidpassing around the outside of the sensing element 12 returns into thepipeline 35 through those holes or slots 41.

Turning now to FIG. 5, there is shown a further embodiment of thepresent invention adapted to be used as a probe, e.g. in large tanks ofliquids where the den- .sity may change with depth. To this end, theapparatus is mounted on a long handle 45 which is secured into anadaptor 46. To the other end of the adaptor 46 is secured thefork-shaped support member or casing 40. For the remainder, theapparatus is substantially similar to that shown in FIG. 4, and it willbe seen that the forkshaped support member 40 is provided with bores toallow the leads from the coils to pass to the amplifier 22 via thehollow interior of the handle 45.

The present invention has so far been described with reference to anelectro-magnetic drive and detector means and the sensing element 12being made of a magnetic material. However, it will be perfectly clearthat other drive means and detector means may be used. Thus the sensingelement 12 may be driven piezoelectrically or magneto-strictively and ingeneral any electro-acoustic transducer may be used to drive it, inwhich case the sensing element may well be made of glass or other suchmaterial adapted to vibrate in response to the electro-acousticexcitement. In a further alternative, the drive may be effected byelectrostatic means, in which case the drive and the detector means maybe constituted by plates for a capacitative drive, the sensing element12 itself constituting the other plate for the capacitors.

While in the above description the sensing element 12 has been describedas a substantially cylindrical tube, its internal cross-section need notnecessarily be circular but may have other shapes.

What I, claim is:

1. Apparatus for measuring the density of a fluid, said apparatusincluding a hollow rigid support member of substantial thickness at onesection thereof within which with a gap therebetween there is mounted ahollow body having open opposite ends and adapted to have said fluidpassing unimpeded straight through its unobstructed interior, externaldrive means for exciting said body from its exterior to vibrate at aresonant frequency, external detecting means for detecting from theexterior of the hollow body a signal representative of the frequency ofsaid vibrations, the frequency of said signal in operation beingdependent substantially only upon the density of said fluid, saidexternal drive means and said external detecting means being disposed ina recess in said section of substantial thickness of the hollow supportmember thereby isolating said drive and detecting means from said fluidand allowing a portion of said fluid to pass externally around theexterior of said hollow body through said gap and in contact therewith,but out of contact with said external drive means and said externaldetecting means, and monitoring means for monitoring said signal.

2. Apparatus as in claim 1 wherein said hollow body is a tube.

3. Apparatus as in claim 1 wherein said hollow body is of substantiallycircular cross-section.

4. Apparatus as in claim 1 wherein said hollow body is provided at eachend thereof with flanges so as to render said body substantiallydumb-bell shaped, whereby to allow said body to be clamped at saidflanges without affecting the frequency of oscillation of the body.

5. Apparatus as in claim 4 wherein said flanges are provided withapertures through which said fluid may pass externally around saidhollow body.

6. Apparatus as in claim 1 wherein both said external drive means andsaid external detecting means are electomagnetic coil means locatedaround the exterior of said hollow body and disposed to substantiallyprevent the flux paths thereof from overlapping each other and whichflux paths are caused to repectively extend into the exterior of saidhollow body over only a small region thereof relative to thecircumference of said body to reduce cross-coupling between the saidexternal drive means and the said external detecting means, theelectromagnetic coil means being 90 out of phase with each other andbeing arranged in planes at right angles to the axis of said hollowbody.

7. Apparatus as in claim 1 wherein said external drive means and saidexternal detecting means are respectively constituted byelectro-acoustic transducers for vibrating the said hollow body.

8. Apparatus as in claim 1 wherein said external drive means and saidexternal detector means are capacitor plates, the exterior of saidhollow body constituting the other plate for the drive capacitor and forthe detector capacitor for the electrostatic excitation of vibrations insaid hollow body.

9. Apparatus as in claim 1 wherein there is provided a locating pin forconnecting said support member to the said hollow body such that removaland replacement of said body may be effected without re-adjusting therelative positions of the said hollow body and the said support member.

10. Apparatus as in claim 1 wherein a core is provided within theinterior of the hollow body so as to reduce the cross-sectional area offluid flow therewithin.

11. Apparatus as in claim 1 wherein said support member is provided withan end portion engageable with an adaptor mounted on a handle for use ofthe apparatus in relatively inaccessible locations.

12. Apparatus as in claim 1 wherein said support member is a rigidcasing surrounding said hollow body along at least the major part of thelength of said hollow body.

13. Apparatus as in claim 1 wherein filter means are provided forfiltering the fluid prior to its passing in contact with said hollowbody.

14. Apparatus for measuring the density of a fluid, comprising a hollowrigid tubular support member through which said fluid is adapted toflow, a hollow tubular body positioned within said support member, saidtubular body having an unobstructed interior and open opposite endspermitting substantially unimpeded flow of fluid therethrough, meansmounting said tubular support member, said mounting means permittingflow of said fluid within said support member and exteriorly of saidtubular body so that said fluid is in contact with and flows across theinner and outer surfaces of said tubular body, external drive means forexciting said tubular body from its exterior to vibrate at a resonantfrequency, external detecting means for detecting from the exterior ofsaid tubular body a signal representative of the frequency of saidvibrations, the frequency of said signal being dependent upon thedensity of said fluid, means mounting said drive means and saiddetecting means at spaced locations exteriorly on said support memberand isolated from said fluid, and monitoring means for monitoring saidsignal.

15. Apparatus as in claim 14 in which said tubular support member has asection thereof of generally cylindrical form and said hollow body is ofgenerally cylindrical form mounted within said cylindrical section ofsaid support member and spaced uniformly from the inner walls of saidsupport member, and wherein said drive means and said detecting meansare mounted on the exterior of the cylindrical section of said supportmember.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,763,692 Dated October 9, 1973 Inventor(s) Agar It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 3, line 18, change "asjustment" to adjustment;

line 51, after "difference" insert between the driving force of coil 19and the detected velocity of the change in the spacing of the element 12and the detector coil 20-;

line 61, before the period insert or a periodic time meter. Column 4,line 1, after "of" insert the positive-;

line 1, after "the", first occurrence, insert detector of pick-up coil20 to the-;

lines 1 and 2, after "19" delete "to the detector or pick-up coil 20";

line 6, after "l2"v insert and;

line 6, change "the" to -its;

line 7, after brations" insert a period and delete "of the sensingelement 12 and";

line 7, change "in" to In-; Column 4, line 9, after "vibrations" changethe comma to a semi-colon line 9, after "say" insert a comma lines 9 and10, after "say" delete "they are natural bell-like vibrations." andinsert one may consider the sensing element 12 similar to two bellsjoined together and clamped at the nodal points (which are the flanges13) As is well-known, clamping a bell at its stem does not affect itsfrequency of oscillation.'-;

line 30, change "of" to e.g.;

line 45, after delete the parenthesis line 45, after "1/" insertparenthesis line 45, before "D" delete parenthesis line 45, after "K"delete the second parenthesis Page #2 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 3, 763, 692 Dated October 9. 1973Inventor(X) Joram Agar It is certified thaterror appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 4, line 49, after "12'' delete the period and insert on thedensity of the material of the sensing element 12 and on the mode ofvibration.-. Column 5, line 3, after "invention" insert by using E-tyleor cup type coils,

line 3, after "of" insert that type of-;

line 3, change "solenoids" to --solenoid-;

line 4, delete "for driving the vibrations because that" and insert as adriver which;

line 14, after "This" insert factor is particularly useful in that it;

line 15, after "of" delete "fluids of high viscosity" and insert liquidsas well as gases-;

lines 31 and 32, after "advantages" delete "in maintaining the phaseshift accurate" and insert accuring from the high efficiency of thedrive and detecting system;

lines 47 and 48, change "amplitude to -zone--.

Signed and Scaled this thir Day Of January 1976 [SEAL] A ttes t:

RUTH C. MASON C. MARSHALL DANN Arms ting Officer Commissioner ufPaIentsand Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIONPatent NO. 317631692 Dated October 9,

Joram Agar Page 1 of 2 Inventor(s) It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3, line 18, change "asjustment" to adjustment;

line 51, after "difference" insert --between the driving force of coil19 and the detected velocity of the change in the spacing of the element12 and the detector coil 20---;

line 61, before the period insert -or a periodic time meter-. Column 4,line 1, after "of" insert --the positive--;

line 1, after "the", first occurrence, insert -detector of pick-up coil20 to the--;

lines 1 and 2, after "19" del t "to the detector or pick-up coil 20";

line 6, after "12" insert --and--;

'line 6, change ''the" to --its-;

line 7, after "brations" insert a period delete "of the sensing element12 and";

,. line 7, change "in" to In-;

Column 4, line 9, after "vibrations" change the coma h) to a semi-colonline 9,

and

after say insert a comma lines 9 and 10, after "say! delete --"they arenatural bell-like vibrations."-- and insert --one may consider thesensing element 12 similar to two bells joined together and clamped atthe nodal points (which are the flanges 13) As is well-known, clamping abell at its stem does not affect its frequency of oscillation.--;

line 30, change "of" to -e.g.--;

line 45, after delete the parenthesis line 45, after "1/" insertparenthesis line 45, before "D' delete parenthesis line 45, after "K"delete the second parenthesis Page 2 of 2 UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent No. 317631692 Dated October 9, 1975Inventorw; Joram Agar It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 4, line 49, after "12" delete the period and insert on thedensity of the material of the sensing element 12 and on the mode ofvibration.-. g Column 5, line 3, after "invention" insert by usingE-tyle or cup type coils,

line 3, after "of" insert ,--that type of--;

line 3, change "solenoids" to --solenoid-;

line 4, delete "for driving the vibrations because that and insert -as adriver which--; 6 line 14, after "This" insert --factor is particularlyuseful in that it--;

line 15, after "of" delete fluids of high viscosity" and insert---liquids as well as gases-;

lines 31 and 32, after "advantages" delete "in maintaining the phaseshift accurate" and insert --accuring Q from the high efficiency of thedrive and detecting system;

lines 47 and 48, change "amplitude to --zone--.

This certificate supersedes Certificate of Correction issued September10, 1974.

Signed and Sealed this tenth Day Of February 1976 [SEAL] Attest: O

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner ufParenIsand Trademarks

1. Apparatus for measuring the density of a fluid, said apparatusincluding a hollow rigid support member of substantial thickness at onesection thereof within which with a gap therebetween there is mounted ahollow body having open opposite ends and adapted to have said fluidpassing unimpeded straight through its unobstructed interior, externaldrive means for exciting said body from its exterior to vibrate at aresonant frequency, external detecting means for detecting from theexterior of the hollow body a signal representative of the frequency ofsaid vibrations, the frequency of said signal in operation beingdependent substantially only upon the density of said fluid, saidexternal drive means and said external detecting means being disposed ina recess in said section of substantial thickness of the hollow supportmember thereby isolating said drive and detecting means from said fluidand allowing a portion of said fluid to pass externally around theexterior of said hollow body through said gap and in contact therewith,but out of contact with said external drive means and said externaldetecting means, and monitoring means for monitoring said signal. 2.Apparatus as in claim 1 wherein said hollow body is a tube.
 3. Apparatusas in claim 1 wherein said hollow body is of substantially circularcross-section.
 4. Apparatus as in claim 1 wherein said hollow body isprovided at each end thereof with flanges so as to render said bodysubstantially dumb-bell shaped, whereby to allow said body to be clampedat said flanges without affecting the frequency of oscillation of thebody.
 5. Apparatus as in claim 4 wherein said flanges are provided withapertures through which said fluid may pass externally around saidhollow body.
 6. Apparatus as in claim 1 wherein both said external drivemeans and said external detecting means are electomagnetic coil meanslocated around the exterior of said hollow body and disposed tosubstantially prevent the flux paths thereof from overlapping each otherand which flux paths are caused to repectively extend into the exteriorof said hollow body over only a small region thereof relative to thecircumference of said body to reduce cross-coupling between the saidexternal drive means and the said external detecting means, theelectromagnetic coil means being 90* out of phase with each other andbeing arranged in planes at right angles to the axis of said hollowbody.
 7. Apparatus as in claim 1 wherein said external drive means andsaid external detecting means are respectively constituted byelectro-acoustic transducers for vibrating the said hollow body. 8.Apparatus as in claim 1 wherein said external drive means and saidexternal detector means are capacitor plates, the exterior of saidhollow body constituting the other plate for the drive capacitor and forthe detector capacitor for the electrostatic excitation of vibrations insaid hollow body.
 9. Apparatus as in claim 1 wherein there is provided alocating pin for connecting said support member to the said hollow bodysuch that removal and replacement of said body may be effected withoutre-adjusting the relative positions of the said hollow body and the saidsupport member.
 10. Apparatus as in claim 1 wherein a core is providedwithin the interior of the hollow body so as to reduce thecross-sectional area of fluid flow therewithin.
 11. Apparatus as inclaim 1 wherein said support member is provided with an end portionengageable with an adaptor mounted on a handle for use of the apparatusin relatively inaccessible locations.
 12. Apparatus as in claim 1wherein said support member is a rigid casing surrounding said hollowbody along at least the major part of the length of said hollow body.13. Apparatus as in claim 1 wherein filter means are provided forfiltering the fluid prior to its passing in contact with said hollowbody.
 14. Apparatus for measuring the density of a fluid, comprising ahollow rigid tubular support member through which said fluid is adaptedto flow, a hollow tubular body positioned within said support member,said tubular body having an unobstructed interior and open opposite endspermitting substantially unimpeded flow of fluid therethrough, meansmounting said tubular support member, said mounting means permittingflow of said fluid within said support member and exteriorly of saidtubular body so that said fluid is in contact with and flows across theinner and outer surfaces of said tubular body, external drive means forexciting said tubular body from its exterior to vibrate at a resonantfrequency, external detecting means for detecting from the exterior ofsaid tubular body a signal representative of the frequency of saidvibrations, the frequency of said signal being dependent upon thedensity of said fluid, means mounting said drive means and saiddetecting means at spaced locations exteriorly on said support memberand isolated from said fluid, and monitoring means for monitoring saidsignal.
 15. Apparatus as in claim 14 in which said tubular supportmember has a section thereof of generally cylindrical form and saidhollow body is of generally cylindrical form mounted within saidcylindrical section of said support member and spaced uniformly from theinner walls of said support member, and wherein said drive means andsaid detecting means are mounted on the exterior of the cylindricalsection of said support member.